At least 80% of the total activity of aspartokinase in crude extracts of Lemna paucicostata was inhibited by lysine, with the remaining activity inhibited by threonine. Inhibition by lysine was synergistically increased by S-adenosylmethionine, which by itself had no effect. Inhibition by lysine and threonine was additive, not cooperative. Aspartokinase activity extracted from Lemna was an order of magnitude greater than that reported by other workers for other plant tissues, and in large excess (approximately 20-fold) of the in vivo requirements for synthesis of the aspartate family of amino acids. Either lysine-sensitive or threonine- sensitive aspartokinase activities alone can support the combined in vivo flux into the aspartate family of amino acids. Severe inhibition of both forms of enzyme activity was required to reduced this flux below the normal requirement. No evidence was obtained for repression/depression of aspartokinase in plants grown with amino acids of the aspartate family. A major conclusion from these combined data is that, contrary to suggestions of other workers, the step catalyzed by aspartokinases does not appear to be the overall rate-limiting one for entry of 4- carbon units into the aspartate pathway. Further, the findings confirm the absence of major "channeling" of lysine-sensitive of threonine-sensitive aspartokinases into separate biosynthetic branches of the aspartate family. The findings also help explain why little, if any, feedback regulation of threonine synthesis occurs in plants supplemented with threonine alone, while complete feedback regulation of threonine synthesis occurs in plants supplemented with both threonine and lysine.